Conventional metal halide discharge lamps include an arc tube having a pair of electrodes mounted therein. The arc tube is mounted in a sealed outer envelope and contains a chemical fill including one or more metal halides. The electrodes are tungsten and commonly include about 0.7% to 2% by weight thorium oxide, or thoria, for increased electron emission from the electrode surfaces. Increased electron emission enhances light output during discharge. In addition, the thoria is radioactive and assists in lamp starting. The starting function is particularly important when the discharge lamp does not include a starting electrode. Low wattage metal halide lamps are disclosed in U.S. Pat. No. 4,620,125 issued Oct. 28, 1986 to Keeffe et al, U.S. Pat. No. 4,625,141 issued Nov. 25, 1986 to Keeffe et al and U.S. Pat. No. 4,415,829 issued Nov. 15, 1983 to Rothwell, Jr. et al.
It has been found that metal halide lamps with electrodes each containing 1% by weight thoria exhibit slow starting. When the thoria content of both electrodes is increased to 2%, the starting times are reduced. However, with 2% thoria content, the electrodes melt back during operation, thereby causing the arc length to be increased. The increased arc length produces increased arc tube operating voltage and wattage, resulting in darkening of the arc tube and loss of luminous efficacy, and sometimes resulting in premature lamp failures. It is desirable to provide metal halide discharge lamps having short starting times and which are not subject to meltback and erosion of the electrodes.
It is a general object of the present invention to provide improved metal halide discharge lamps.
It is a another object of the present invention to provide metal halide discharge lamps having fast starting characteristics.
It is a further object of the present invention to provide metal halide discharge lamps wherein electrode meltback and erosion during operation are limited.
It is a further object of the present invention to provide metal halide discharge lamps having long operating lives.
It is yet another object of the present invention to provide metal halide discharge lamps having essentially constant operating characteristics as a function of time.